1. Field of the Invention
The present invention relates to a motor driver furnished with a function of controlling the driving of a motor, such as a stepping motor or spindle motor, through negative feedback control of the current supplied to the motor.
2. Description of the Prior Art
FIG. 13 shows the circuit configuration of a conventional, typical stepping motor driver. Reference numerals 401A and 401B represent output circuits. Reference numeral 402A represents an operational amplifier. Reference numeral 402B represents a direct-current voltage source. Reference numeral 402C represents a controller. Reference numeral 402D represents an externally fitted resistor. Reference numerals 403A and 403B represent coils of a stepping motor. Reference numerals 404A, 404B, 404C, 404D, 404E, 404F, 404G, and 404H represent NPN-type transistors. Here, it is assumed that the stepping motor driver shown in FIG. 13 is for driving a stepping motor having two coils.
The output circuit 401A is composed of four transistors 404A, 404B, 404C, and 404D. The output circuit 401B is composed of four transistors 404E, 404F, 404G, and 404H. The collectors of the transistors 404A, 404C, 404E, and 404G are connected together to a terminal 405A. The terminal 405A is connected to a supply voltage VCC. The emitters of the transistors 404B, 404D, 404F, and 404H are connected together to a terminal 405B. The terminal 405B is connected through the resistor 402D to ground GND.
In the output circuit 401A, the emitter of the transistor 404A and the collector of the transistor 404B are connected together to a terminal 405C, and the emitter of the transistor 404C and the collector of the transistor 404D are connected together to a terminal 405D. Between the terminals 405C and 405D, one coil 403A of the stepping motor is connected.
In the output circuit 401B, the emitter of the transistor 404E and the collector of the transistor 404F are connected together to a terminal 405E, and the emitter of the transistor 404G and the collector of the transistor 404H are connected together to a terminal 405F. Between the terminals 405E and 405F, the other coil 403B of the stepping motor is connected.
A direct-current voltage generated by the voltage source 402B is fed to the non-inverting input terminal (+) of the operational amplifier 402A, of which the inverting input terminal (xe2x88x92) is connected to the terminal 405B so that all the current that flows through the stepping motor is converted into a voltage by the resistor 402D and is then fed to the inverting input terminal (xe2x88x92).
The controller 402C controls the turning on/off of the individual transistors constituting the output circuits 401A and 401B in such a way that the direction of the current that flows through the coils 403A and 403B is switched with appropriate timing. Moreover, the controller 402C controls the current fed to the bases of the individual transistors constituting the output circuits 401A and 401B when those transistors are turned on in such a way that the output of the operational amplifier 402A does not fall below a reference level. In this way, negative feedback control is achieved in such a way that the current flowing through the stepping motor as a whole does not exceed a predetermined value.
However, in cases like this, where the current flowing through a stepping motor is controlled through negative feedback, depending on the specifications of the stepping motor actually driven, the following problems may arise. As shown in FIG. 14, when the transistors constituting the output circuits are turned on/off, the inductance present in the coils of the stepping motor may cause overshooting or oscillation in the current I flowing through the coils of the stepping motor, or oscillation lasting for several microseconds to several milliseconds in the voltages VA and VB across the coils, both leading to increased noise. This often causes problems in particular in electronic devices such as floppy disk drives that comply with the USB standard, because such devices receive electric power from their host and thus need to meet strict requirements with respect to the current that they consume.
Moreover, there is a type of problem in which, although an abnormal current is flowing through the coil of one phase of the stepping motor, the current flowing through the motor as a whole appears quite normal. In such a case, with the conventional stepping motor driver described above, which controls the current flowing through the motor as a whole with a single negative feedback circuit, it is not possible to overcome the problem, and thus it is not possible to drive the stepping motor with satisfactory accuracy.
Moreover, since a current so high as all the current supplied to the stepping motor is controlled through negative feedback, the resistances of the conductors that connect the output circuits to the supply power terminals affect the feedback factor greatly. Thus, problems tend to result from the fact that the resistances of conductors vary from one IC chip to another.
FIG. 15 shows a plan view and a connection diagram of the conventional stepping motor driver described above. As this figure shows, the output circuits 401A and 401B are connected by way of a typical conductor 407 to the terminal 405B, which in turn is connected, outside the stepping motor driver, through the resistor 402D to ground VGND. However, the two output circuits 401A and 401B use different lengths of this conductor 407, and thus the resistance of the conductor 407 from the output circuits 401A and 401B to the terminal 405B differs between them, with the result that the currents flowing through the coils 403A and 403B differ significantly from each other. Furthermore, the switching noise that is generated in the output circuits 401A and 401B tends to affect a current limiter circuit 406A and other internal circuits adversely. This often causes problems in particular in electronic devices that comply with the USB standard, because such devices need to meet strict requirements with respect to the current that they consume.
An object of the present invention is to provide a motor driver that is furnished with a function of controlling the current supplied to the coils of a motor through negative feedback and that is designed to minimize the noise accompanying the switching operations performed in output circuits.
Another object of the present invention is to provide a motor driver that can drive a motor such as a stepping motor with satisfactory accuracy.
Another object of the present invention is to provide a motor driver that is less prone to problems resulting from the fact that the resistances of conductors vary from one IC chip to another.
Another object of the present invention is to provide a motor driver that can supply individual coils of a motor with as equal currents as possible.
Another object of the present invention is to provide a motor driver that can minimize the adverse effect of the switching noise generated in output circuits on other internal circuits.
To achieve the above objects, according to one aspect of the present invention, in a motor driver furnished with a function of controlling the driving of a motor through negative feedback control of the current supplied to the coil of the motor, the motor driver permits the setting of a target value of the current that is negatively fed back, and sets the target value in such a way that the target value at a time point when the current starts being supplied to the coil of the motor is smaller than the target value after the current starts being supplied to the coil of the motor.
In this circuit configuration, the current that flows through the coil increases more gradually than in conventional circuit configurations in which the target value is fixed at the maximum value thereof before the current starts being supplied to the coil of the motor. This suppresses overshooting and oscillation in the current flowing through the coil and oscillation in the voltage across the coil, and thereby reduces noise.
According to another aspect of the present invention, in a motor driver furnished with a function of controlling the driving of a motor through negative feedback control of the currents supplied to the motor, the currents flowing through the coils of different phases of the motor are negatively fed back individually.
This circuit configuration is free from the type of problem in which, although the current flowing through the motor as a whole appears normal, actually an abnormal current is flowing through the coil of one phase of the motor as experienced in cases where negative feedback control is achieved on the basis of the current flowing through the motor as a whole. As a result, it is possible to drive the motor with satisfactory accuracy. Moreover, it is also possible to minimize the effect of the resistances of conductors on the feedback factor, and thereby reduce problems resulting from the fact that the resistances of conductors vary from one IC chip to another.
According to another aspect of the present invention, in a motor driver furnished with a function of controlling the driving of a motor through negative feedback control of the currents supplied to the motor, damping devices are built in between the terminals to which the coils of the motor are connected.
In this circuit configuration, the currents that flow through the coils of the motor vary more gradually than in a circuit configuration without damping devices. This suppresses overshooting and oscillation in the currents flowing through the coils and oscillation in the voltages across the coils, and thereby reduces noise.